This invention concerns improvements in or relating to head-up displays.
A head-up display system presents to an observer, usually the pilot of an aircraft, a view of displayed visual information which is superimposed on the observer's view of the outside scene. The super-imposition can be achieved by means of a partially transparent reflector, often referred to as a combiner, through which the outside scene is viewed and which reflects light from a display source towards the observer's eyes. There have also been proposals for using diffractive optics combiners, and in particular holographic combiners, through which the observer views the outside scene and which deviate light from the display source either in a reflective mode or in a transmissive mode towards the observer's eyes. Such diffractive optics combiners generally have a real or an effective exit pupil within whose limits the observer's eyes must view in order to see the displayed visual information.
In some circumstances, and particularly for training purposes, there is a requirement for a record and/or a transmission of the view of the outside scene and the superimposed displayed visual information. In particular, there is a requirement for being able to evaluate, analyse and if necessary criticise a trainee pilot's actions and reactions relative to such view. Such record and/or transmission can be provided by means of a camera, which may be a film or cine camera or a television camera, but there are various problems associated with the provision of a satisfactory camera arrangement. These problems include the need to avoid interference with or obstruction of the pilot's view through the head-up display, or undue complication of the head-up display system. Also in an aircraft cockpit there are usually severe space limitations which make it difficult to find a suitable and safe location for the camera arrangement.
Some of these problems are outlined in U.S. Pat. No. 4,001,499 which suggests the use of a television camera mounted outside the pilot's cockpit on the nose, fuselage or a wing of the aircraft, or conceivably mounted within the cockpit but not viewing through the combiner, and an arrangement for combining video signals of the outside scene viewed by the camera with video signals derived from the display. A problem with this, and other arrangements involving a separate system from the head-up display system through which the pilot views, is that the outputs of the two systems may not always correspond. In particular there may sometimes be relative misalignment between the systems, or there may be failure or malfunction of a part in one system whose effect is not apparent in the other. This can therefore affect the validity of analysis in that it is not certain that the record or transmission on which the analysis is based in fact corresponds to the pilot's actual view (and a trainee pilot can contend that criticism of his actions or reactions is unjustified because it is based on something different from what he actually saw).